
                           Tor Path Specification

                              Roger Dingledine
                               Nick Mathewson

Note: This is an attempt to specify Tor as currently implemented.  Future
versions of Tor will implement improved algorithms.

This document tries to cover how Tor chooses to build circuits and assign
streams to circuits.  Other implementations MAY take other approaches, but
implementors should be aware of the anonymity and load-balancing implications
of their choices.

                    THIS SPEC ISN'T DONE YET.

      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
      "OPTIONAL" in this document are to be interpreted as described in
      RFC 2119.

1. General operation

   Tor begins building circuits as soon as it has enough directory
   information to do so (see section 5 of dir-spec.txt).  Some circuits are
   built preemptively because we expect to need them later (for user
   traffic), and some are built because of immediate need (for user traffic
   that no current circuit can handle, for testing the network or our
   reachability, and so on).

  [Newer versions of Tor (0.2.6.2-alpha and later):
   If the consensus contains Exits (the typical case), Tor will build both
   exit and internal circuits. When bootstrap completes, Tor will be ready
   to handle an application requesting an exit circuit to services like the
   World Wide Web.

   If the consensus does not contain Exits, Tor will only build internal
   circuits. In this case, earlier statuses will have included "internal"
   as indicated above. When bootstrap completes, Tor will be ready to handle
   an application requesting an internal circuit to hidden services at
   ".onion" addresses.

   If a future consensus contains Exits, exit circuits may become available.]

   When a client application creates a new stream (by opening a SOCKS
   connection or launching a resolve request), we attach it to an appropriate
   open circuit if one exists, or wait if an appropriate circuit is
   in-progress. We launch a new circuit only
   if no current circuit can handle the request.  We rotate circuits over
   time to avoid some profiling attacks.

   To build a circuit, we choose all the nodes we want to use, and then
   construct the circuit.  Sometimes, when we want a circuit that ends at a
   given hop, and we have an appropriate unused circuit, we "cannibalize" the
   existing circuit and extend it to the new terminus.

   These processes are described in more detail below.

   This document describes Tor's automatic path selection logic only; path
   selection can be overridden by a controller (with the EXTENDCIRCUIT and
   ATTACHSTREAM commands).  Paths constructed through these means may
   violate some constraints given below.

1.1. Terminology

   A "path" is an ordered sequence of nodes, not yet built as a circuit.

   A "clean" circuit is one that has not yet been used for any traffic.

   A "fast" or "stable" or "valid" node is one that has the 'Fast' or
   'Stable' or 'Valid' flag
   set respectively, based on our current directory information.  A "fast"
   or "stable" circuit is one consisting only of "fast" or "stable" nodes.

   In an "exit" circuit, the final node is chosen based on waiting stream
   requests if any, and in any case it avoids nodes with exit policy of
   "reject *:*". An "internal" circuit, on the other hand, is one where
   the final node is chosen just like a middle node (ignoring its exit
   policy).

   A "request" is a client-side stream or DNS resolve that needs to be
   served by a circuit.

   A "pending" circuit is one that we have started to build, but which has
   not yet completed.

   A circuit or path "supports" a request if it is okay to use the
   circuit/path to fulfill the request, according to the rules given below.
   A circuit or path "might support" a request if some aspect of the request
   is unknown (usually its target IP), but we believe the path probably
   supports the request according to the rules given below.

1.1. A relay's bandwidth

   Old versions of Tor did not report bandwidths in network status
   documents, so clients had to learn them from the routers' advertised
   relay descriptors.

   For versions of Tor prior to 0.2.1.17-rc, everywhere below where we
   refer to a relay's "bandwidth", we mean its clipped advertised
   bandwidth, computed by taking the smaller of the 'rate' and
   'observed' arguments to the "bandwidth" element in the relay's
   descriptor.  If a router's advertised bandwidth is greater than
   MAX_BELIEVABLE_BANDWIDTH (currently 10 MB/s), we clipped to that
   value.

   For more recent versions of Tor, we take the bandwidth value declared
   in the consensus, and fall back to the clipped advertised bandwidth
   only if the consensus does not have bandwidths listed.

2. Building circuits

2.1. When we build

2.1.0. We don't build circuits until we have enough directory info

   There's a class of possible attacks where our directory servers
   only give us information about the relays that they would like us
   to use.  To prevent this attack, we don't build multi-hop
   circuits for real traffic (like those in 2.1.1, 2.1.2, 2.1.4
   below) until we have enough directory information to be
   reasonably confident this attack isn't being done to us.

   Here, "enough" directory information is defined as:

      * Having a consensus that's been valid at some point in the
        last REASONABLY_LIVE_TIME interval (24 hours).

      * Having enough descriptors that we could build at least some
        fraction F of all bandwidth-weighted paths, without taking
        ExitNodes/EntryNodes/etc into account.

        (F is set by the PathsNeededToBuildCircuits option,
        defaulting to the 'min_paths_for_circs_pct' consensus
        parameter, with a final default value of 60%.)

      * Having enough descriptors that we could build at least some
        fraction F of all bandwidth-weighted paths, _while_ taking
        ExitNodes/EntryNodes/etc into account.

        (F is as above.)

      * Having a descriptor for every one of the first
        NUM_GUARDS_TO_USE guards among our primary guards. (see
        guard-spec.txt)


2.1.1. Clients build circuits preemptively

   When running as a client, Tor tries to maintain at least a certain
   number of clean circuits, so that new streams can be handled
   quickly.  To increase the likelihood of success, Tor tries to
   predict what circuits will be useful by choosing from among nodes
   that support the ports we have used in the recent past (by default
   one hour). Specifically, on startup Tor tries to maintain one clean
   fast exit circuit that allows connections to port 80, and at least
   two fast clean stable internal circuits in case we get a resolve
   request or hidden service request (at least three if we _run_ a
   hidden service).

   After that, Tor will adapt the circuits that it preemptively builds
   based on the requests it sees from the user: it tries to have two fast
   clean exit circuits available for every port seen within the past hour
   (each circuit can be adequate for many predicted ports -- it doesn't
   need two separate circuits for each port), and it tries to have the
   above internal circuits available if we've seen resolves or hidden
   service activity within the past hour. If there are 12 or more clean
   circuits open, it doesn't open more even if it has more predictions.

   Only stable circuits can "cover" a port that is listed in the
   LongLivedPorts config option. Similarly, hidden service requests
   to ports listed in LongLivedPorts make us create stable internal
   circuits.

   Note that if there are no requests from the user for an hour, Tor
   will predict no use and build no preemptive circuits.

   The Tor client SHOULD NOT store its list of predicted requests to a
   persistent medium.

2.1.2. Clients build circuits on demand

   Additionally, when a client request exists that no circuit (built or
   pending) might support, we create a new circuit to support the request.
   For exit connections, we pick an exit node that will handle the
   most pending requests (choosing arbitrarily among ties), launch a
   circuit to end there, and repeat until every unattached request
   might be supported by a pending or built circuit. For internal
   circuits, we pick an arbitrary acceptable path, repeating as needed.

   In some cases we can reuse an already established circuit if it's
   clean; see Section 2.3 (cannibalizing circuits) for details.

2.1.3. Relays build circuits for testing reachability and bandwidth

   Tor relays test reachability of their ORPort once they have
   successfully built a circuit (on startup and whenever their IP address
   changes). They build an ordinary fast internal circuit with themselves
   as the last hop. As soon as any testing circuit succeeds, the Tor
   relay decides it's reachable and is willing to publish a descriptor.

   We launch multiple testing circuits (one at a time), until we
   have NUM_PARALLEL_TESTING_CIRC (4) such circuits open. Then we
   do a "bandwidth test" by sending a certain number of relay drop
   cells down each circuit: BandwidthRate * 10 / CELL_NETWORK_SIZE
   total cells divided across the four circuits, but never more than
   CIRCWINDOW_START (1000) cells total. This exercises both outgoing and
   incoming bandwidth, and helps to jumpstart the observed bandwidth
   (see dir-spec.txt).

   Tor relays also test reachability of their DirPort once they have
   established a circuit, but they use an ordinary exit circuit for
   this purpose.

2.1.4. Hidden-service circuits

   See section 4 below.

2.1.5. Rate limiting of failed circuits

   If we fail to build a circuit N times in a X second period (see Section
   2.3 for how this works), we stop building circuits until the X seconds
   have elapsed.
   XXXX

2.1.6. When to tear down circuits

   XXXX


2.2. Path selection and constraints

   We choose the path for each new circuit before we build it.  We choose the
   exit node first, followed by the other nodes in the circuit.  All paths
   we generate obey the following constraints:

     - We do not choose the same router twice for the same path.
     - We do not choose any router in the same family as another in the same
       path. (Two routers are in the same family if each one lists the other
       in the "family" entries of its descriptor.)
     - We do not choose more than one router in a given /16 subnet
       (unless EnforceDistinctSubnets is 0).
     - We don't choose any non-running or non-valid router unless we have
       been configured to do so. By default, we are configured to allow
       non-valid routers in "middle" and "rendezvous" positions.
     - If we're using Guard nodes, the first node must be a Guard (see 5
       below)
     - XXXX Choosing the length

   For "fast" circuits, we only choose nodes with the Fast flag. For
   non-"fast" circuits, all nodes are eligible.

   For all circuits, we weight node selection according to router bandwidth.

   We also weight the bandwidth of Exit and Guard flagged nodes depending on
   the fraction of total bandwidth that they make up and depending upon the
   position they are being selected for.

   These weights are published in the consensus, and are computed as described
   in Section "Computing Bandwidth Weights" of dir-spec.txt. They are:

      Wgg - Weight for Guard-flagged nodes in the guard position
      Wgm - Weight for non-flagged nodes in the guard Position
      Wgd - Weight for Guard+Exit-flagged nodes in the guard Position

      Wmg - Weight for Guard-flagged nodes in the middle Position
      Wmm - Weight for non-flagged nodes in the middle Position
      Wme - Weight for Exit-flagged nodes in the middle Position
      Wmd - Weight for Guard+Exit flagged nodes in the middle Position

      Weg - Weight for Guard flagged nodes in the exit Position
      Wem - Weight for non-flagged nodes in the exit Position
      Wee - Weight for Exit-flagged nodes in the exit Position
      Wed - Weight for Guard+Exit-flagged nodes in the exit Position

      Wgb - Weight for BEGIN_DIR-supporting Guard-flagged nodes
      Wmb - Weight for BEGIN_DIR-supporting non-flagged nodes
      Web - Weight for BEGIN_DIR-supporting Exit-flagged nodes
      Wdb - Weight for BEGIN_DIR-supporting Guard+Exit-flagged nodes

      Wbg - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
      Wbm - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
      Wbe - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
      Wbd - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests

   If any of those weights is malformed or not present in a consensus,
   clients proceed with the regular path selection algorithm setting
   the weights to the default value of 10000.

   Additionally, we may be building circuits with one or more requests in
   mind.  Each kind of request puts certain constraints on paths:

     - All service-side introduction circuits and all rendezvous paths
       should be Stable.
     - All connection requests for connections that we think will need to
       stay open a long time require Stable circuits.  Currently, Tor decides
       this by examining the request's target port, and comparing it to a
       list of "long-lived" ports. (Default: 21, 22, 706, 1863, 5050,
       5190, 5222, 5223, 6667, 6697, 8300.)
     - DNS resolves require an exit node whose exit policy is not equivalent
       to "reject *:*".
     - Reverse DNS resolves require a version of Tor with advertised eventdns
       support (available in Tor 0.1.2.1-alpha-dev and later).
     - All connection requests require an exit node whose exit policy
       supports their target address and port (if known), or which "might
       support it" (if the address isn't known).  See 2.2.1.
     - Rules for Fast? XXXXX

2.2.1. Choosing an exit

   If we know what IP address we want to connect to or resolve, we can
   trivially tell whether a given router will support it by simulating
   its declared exit policy.

   Because we often connect to addresses of the form hostname:port, we do not
   always know the target IP address when we select an exit node.  In these
   cases, we need to pick an exit node that "might support" connections to a
   given address port with an unknown address.  An exit node "might support"
   such a connection if any clause that accepts any connections to that port
   precedes all clauses (if any) that reject all connections to that port.

   Unless requested to do so by the user, we never choose an exit node
   flagged as "BadExit" by more than half of the authorities who advertise
   themselves as listing bad exits.

2.2.2. User configuration

   Users can alter the default behavior for path selection with configuration
   options.

   - If "ExitNodes" is provided, then every request requires an exit node on
     the ExitNodes list.  (If a request is supported by no nodes on that list,
     and StrictExitNodes is false, then Tor treats that request as if
     ExitNodes were not provided.)

   - "EntryNodes" and "StrictEntryNodes" behave analogously.

   - If a user tries to connect to or resolve a hostname of the form
     <target>.<servername>.exit, the request is rewritten to a request for
     <target>, and the request is only supported by the exit whose nickname
     or fingerprint is <servername>.

   - When set, "HSLayer2Nodes" and "HSLayer3Nodes" relax Tor's path
     restrictions to allow nodes in the same /16 and node family to reappear
     in the path. They also allow the guard node to be chosen as the RP, IP,
     and HSDIR, and as the hop before those positions.

2.3. Cannibalizing circuits

   If we need a circuit and have a clean one already established, in
   some cases we can adapt the clean circuit for our new
   purpose. Specifically,

   For hidden service interactions, we can "cannibalize" a clean internal
   circuit if one is available, so we don't need to build those circuits
   from scratch on demand.

   We can also cannibalize clean circuits when the client asks to exit
   at a given node -- either via the ".exit" notation or because the
   destination is running at the same location as an exit node.

2.4. Learning when to give up ("timeout") on circuit construction

   Since version 0.2.2.8-alpha, Tor clients attempt to learn when to give
   up on circuits based on network conditions.

2.4.1. Distribution choice

   Based on studies of build times, we found that the distribution of
   circuit build times appears to be a Frechet distribution (and a multi-modal
   Frechet distribution, if more than one guard or bridge is used). However,
   estimators and quantile functions of the Frechet distribution are difficult
   to work with and slow to converge. So instead, since we are only interested
   in the accuracy of the tail, clients approximate the tail of the multi-modal
   distribution with a single Pareto curve.

2.4.2. How much data to record

   From our observations, the minimum number of circuit build times for a
   reasonable fit appears to be on the order of 100. However, to keep a
   good fit over the long term, clients store 1000 most recent circuit build
   times in a circular array.

   The Tor client should build test circuits at a rate of one every 'cbttestfreq'
   (10 seconds) until 'cbtmincircs' (100 circuits) are built, with a maximum of
   'cbtmaxopencircs' (default: 10) circuits open at once. This allows a fresh
   Tor to have a CircuitBuildTimeout estimated within 30 minutes after install
   or network change (see section 2.4.5 below).

   Timeouts are stored on disk in a histogram of 10ms bin width, the same
   width used to calculate the Xm value above. This histogram must be shuffled
   after being read from disk, to preserve a proper expiration of old values
   after restart.

   Thus, some build time resolution is lost during restart. Implementations may
   choose a different persistence mechanism than this histogram, but be aware
   that build time binning is still needed for parameter estimation.

2.4.3. Parameter estimation

   Once 'cbtmincircs' build times are recorded, Tor clients update the
   distribution parameters and recompute the timeout every circuit completion
   (though see section 2.4.5 for when to pause and reset timeout due to
   network change or connectivity loss).

   Tor clients calculate the parameters for a Pareto distribution fitting the
   data using the maximum likelihood estimator. For derivation, see:
   https://en.wikipedia.org/wiki/Pareto_distribution#Estimation_of_parameters

   Because build times are not a true Pareto distribution, we alter how Xm is
   computed. In a max likelihood estimator, the mode of the distribution is
   used directly as Xm.

   Instead of using the mode of discrete build times directly, Tor clients
   compute the Xm parameter using the weighted average of the the midpoints
   of the 'cbtnummodes' (10) most frequently occurring 10ms histogram bins.
   (The use of 10 modes was found to minimize error from the selected
   cbtquantile, with 10ms bins for quantiles 60-80, compared to many other
   heuristics).

   To avoid ln(1.0+epsilon) precision issues, use log laws to rewrite the
   estimator for 'alpha' as the sum of logs followed by subtraction, rather
   than multiplication and division:

       alpha = n/(Sum_n{ln(MAX(Xm, x_i))} - n*ln(Xm))

   In this, n is the total number of build times that have completed, x_i is
   the ith recorded build time, and Xm is the modes of x_i as above.

   All times below Xm are counted as having the Xm value via the MAX(),
   because in Pareto estimators, Xm is supposed to be the lowest value.
   However, since clients use mode averaging to estimatre Xm, there can be
   values below our Xm. Effectively, the Pareto estimator then treats that
   everything smaller than Xm happened at Xm. One can also see that if
   clients did not do this, alpha could underflow to become negative, which
   results in an exponential curve, not a Pareto probability distribution.

   The timeout itself is calculated by using the Pareto Quantile function (the
   inverted CDF) to give us the value on the CDF such that 80% of the mass
   of the distribution is below the timeout value (parameter 'cbtquantile').

   The Pareto Quantile Function (inverse CDF) is:

      F(q) = Xm/((1.0-q)^(1.0/alpha))

   Thus, clients obtain their circuit build timeout by computing:

     timeout_ms = F(0.8)    # 'cbtquantile' == 0.8

   With this, we expect that the Tor client will accept the fastest 80% of the
   total number of paths on the network.

   Clients obtain the circuit close time to completely abandon circuits as:

     close_ms = F(0.99)     # 'cbtclosequantile' == 0.99

   To avoid waiting an unreasonably long period of time for circuits that
   simply have relays that are down, Tor clients cap timeout_ms at the max
   build time actually observed so far, and cap close_ms at twice this max,
   but at least 60 seconds:

     timeout_ms = MIN(timeout_ms, max_observed_timeout)
     close_ms = MAX(MIN(close_ms, 2*max_observed_timeout), 'cbtinitialtimeout')

2.4.4. How to record timeouts

   Pareto estimators begin to lose their accuracy if the tail is omitted.
   Hence, Tor clients actually calculate two timeouts: a usage timeout, and a
   close timeout.

   Circuits that pass the usage timeout are marked as measurement circuits,
   and are allowed to continue to build until the close timeout corresponding
   to the point 'cbtclosequantile' (default 99) on the Pareto curve, or 60
   seconds, whichever is greater.

   The actual completion times for these measurements circuits should be
   recorded.

   Implementations should completely abandon a circuit and ignore the circuit
   if the total build time exceeds the close threshold. Such closed circuits
   should be ignored, as this typically means one of the relays in the path is
   offline.

2.4.5. Detecting Changing Network Conditions

   Tor clients attempt to detect both network connectivity loss and drastic
   changes in the timeout characteristics.

   Clients assume that they have had network connectivity loss if a circuit
   times out and have received no cells or TLS handshakes since that
   circuit began. Clients then temporarily stop counting timeouts until
   network activity resumes (ie: until a TLS handshake completes or a cell
   arrives at the client).

   To detect changing network conditions, clients keep a history of
   the timeout or non-timeout status of the past 'cbtrecentcount' circuits
   (20 circuits) that successfully completed at least one hop. If more than
   90% of these circuits timeout, the client discards all buildtimes history,
   resets the timeout to 'cbtinitialtimeout' (60 seconds), and then begins
   recomputing the timeout.

   If the timeout was already 60 or higher, the client doubles the timeout.

2.4.6. Consensus parameters governing behavior

   Clients that implement circuit build timeout learning should obey the
   following consensus parameters that govern behavior, in order to allow
   us to handle bugs or other emergent behaviors due to client circuit
   construction. If these parameters are not present in the consensus,
   the listed default values should be used instead.

      cbtdisabled
        Default: 0
        Min: 0
        Max: 1
        Effect: If 1, all CircuitBuildTime learning code should be
                disabled and history should be discarded. For use in
                emergency situations only.

      cbtnummodes
        Default: 10
        Min: 1
        Max: 20
        Effect: This value governs how many modes to use in the weighted
        average calculation of Pareto parameter Xm. Selecting Xm as the
        average of multiple modes improves accuracy of the Pareto tail
        for quantile cutoffs from 60-80% (see cbtquantile).

      cbtrecentcount
        Default: 20
        Min: 3
        Max: 1000
        Effect: This is the number of circuit build times to keep track of
                for the following option.

      cbtmaxtimeouts
        Default: 18
        Min: 3
        Max: 10000
        Effect: When this many timeouts happen in the last 'cbtrecentcount'
                circuit attempts, the client should discard all of its
                history and begin learning a fresh timeout value.

      cbtmincircs
        Default: 100
        Min: 1
        Max: 10000
        Effect: This is the minimum number of circuits to build before
                computing a timeout.

      cbtquantile
        Default: 80
        Min: 10
        Max: 99
        Effect: This is the position on the quantile curve to use to set the
                timeout value. It is a percent (10-99).

      cbtclosequantile
        Default: 99
        Min: Value of cbtquantile parameter
        Max: 99
        Effect: This is the position on the quantile curve to use to set the
                timeout value to use to actually close circuits. It is a
                percent (0-99).

      cbttestfreq
        Default: 10
        Min: 1
        Max: 2147483647 (INT32_MAX)
        Effect: Describes how often in seconds to build a test circuit to
                gather timeout values. Only applies if less than 'cbtmincircs'
                have been recorded.

      cbtmintimeout
        Default: 10
        Min: 10
        Max: 2147483647 (INT32_MAX)
        Effect: This is the minimum allowed timeout value in milliseconds.

      cbtinitialtimeout
        Default: 60000
        Min: Value of cbtmintimeout
        Max: 2147483647 (INT32_MAX)
        Effect: This is the timeout value to use before computing a timeout,
                in milliseconds.

      cbtlearntimeout
        Default: 180
        Min: 10
        Max: 60000
        Effect: This is how long idle circuits will be kept open while cbt is
                learning a new timeout value.

      cbtmaxopencircs
        Default: 10
        Min: 0
        Max: 14
        Effect: This is the maximum number of circuits that can be open at
                at the same time during the circuit build time learning phase.

2.5. Handling failure

   If an attempt to extend a circuit fails (either because the first create
   failed or a subsequent extend failed) then the circuit is torn down and is
   no longer pending.  (XXXX really?)  Requests that might have been
   supported by the pending circuit thus become unsupported, and a new
   circuit needs to be constructed.

   If a stream "begin" attempt fails with an EXITPOLICY error, we
   decide that the exit node's exit policy is not correctly advertised,
   so we treat the exit node as if it were a non-exit until we retrieve
   a fresh descriptor for it.

   Excessive amounts of either type of failure can indicate an
   attack on anonymity. See section 7 for how excessive failure is handled.

3. Attaching streams to circuits

   When a circuit that might support a request is built, Tor tries to attach
   the request's stream to the circuit and sends a BEGIN, BEGIN_DIR,
   or RESOLVE relay
   cell as appropriate.  If the request completes unsuccessfully, Tor
   considers the reason given in the CLOSE relay cell. [XXX yes, and?]


   After a request has remained unattached for SocksTimeout (2 minutes
   by default), Tor abandons the attempt and signals an error to the
   client as appropriate (e.g., by closing the SOCKS connection).

   XXX Timeouts and when Tor auto-retries.

    * What stream-end-reasons are appropriate for retrying.

   If no reply to BEGIN/RESOLVE, then the stream will timeout and fail.

4. Hidden-service related circuits

  XXX Tracking expected hidden service use (client-side and hidserv-side)

5. Guard nodes

  We use Guard nodes (also called "helper nodes" in the research
  literature) to prevent certain profiling attacks. For an overview of
  our Guard selection algorithm -- which has grown rather complex -- see
  guard-spec.txt.

5.1. How consensus bandwidth weights factor into entry guard selection

  When weighting a list of routers for choosing an entry guard, the following
  consensus parameters (from the "bandwidth-weights" line) apply:

      Wgg - Weight for Guard-flagged nodes in the guard position
      Wgm - Weight for non-flagged nodes in the guard Position
      Wgd - Weight for Guard+Exit-flagged nodes in the guard Position
      Wgb - Weight for BEGIN_DIR-supporting Guard-flagged nodes
      Wmb - Weight for BEGIN_DIR-supporting non-flagged nodes
      Web - Weight for BEGIN_DIR-supporting Exit-flagged nodes
      Wdb - Weight for BEGIN_DIR-supporting Guard+Exit-flagged nodes

  Please see "bandwidth-weights" in §3.4.1 of dir-spec.txt for more in depth
  descriptions of these parameters.

  If a router has been marked as both an entry guard and an exit, then we
  prefer to use it more, with our preference for doing so (roughly) linearly
  increasing w.r.t. the router's non-guard bandwidth and bandwidth weight
  (calculated without taking the guard flag into account).  From proposal
  #236:

    |
    | Let Wpf denote the weight from the 'bandwidth-weights' line a
    | client would apply to N for position p if it had the guard
    | flag, Wpn the weight if it did not have the guard flag, and B the
    | measured bandwidth of N in the consensus.  Then instead of choosing
    | N for position p proportionally to Wpf*B or Wpn*B, clients should
    | choose N proportionally to F*Wpf*B + (1-F)*Wpn*B.

  where F is the weight as calculated using the above parameters.

6. Server descriptor purposes

  There are currently three "purposes" supported for server descriptors:
  general, controller, and bridge. Most descriptors are of type general
  -- these are the ones listed in the consensus, and the ones fetched
  and used in normal cases.

  Controller-purpose descriptors are those delivered by the controller
  and labelled as such: they will be kept around (and expire like
  normal descriptors), and they can be used by the controller in its
  CIRCUITEXTEND commands. Otherwise they are ignored by Tor when it
  chooses paths.

  Bridge-purpose descriptors are for routers that are used as bridges. See
  doc/design-paper/blocking.pdf for more design explanation, or proposal
  125 for specific details. Currently bridge descriptors are used in place
  of normal entry guards, for Tor clients that have UseBridges enabled.

7. Detecting route manipulation by Guard nodes (Path Bias)

  The Path Bias defense is designed to defend against a type of route
  capture where malicious Guard nodes deliberately fail or choke circuits
  that extend to non-colluding Exit nodes to maximize their network
  utilization in favor of carrying only compromised traffic.

  In the extreme, the attack allows an adversary that carries c/n
  of the network capacity to deanonymize c/n of the network
  connections, breaking the O((c/n)^2) property of Tor's original
  threat model. It also allows targeted attacks aimed at monitoring
  the activity of specific users, bridges, or Guard nodes.

  There are two points where path selection can be manipulated:
  during construction, and during usage. Circuit construction
  can be manipulated by inducing circuit failures during circuit
  extend steps, which causes the Tor client to transparently retry
  the circuit construction with a new path. Circuit usage can be
  manipulated by abusing the stream retry features of Tor (for
  example by withholding stream attempt responses from the client
  until the stream timeout has expired), at which point the tor client
  will also transparently retry the stream on a new path.

  The defense as deployed therefore makes two independent sets of
  measurements of successful path use: one during circuit construction,
  and one during circuit usage.

  The intended behavior is for clients to ultimately disable the use
  of Guards responsible for excessive circuit failure of either type
  (see section 7.4); however known issues with the Tor network currently
  restrict the defense to being informational only at this stage (see
  section 7.5).

7.1. Measuring path construction success rates

  Clients maintain two counts for each of their guards: a count of the
  number of times a circuit was extended to at least two hops through that
  guard, and a count of the number of circuits that successfully complete
  through that guard. The ratio of these two numbers is used to determine
  a circuit success rate for that Guard.

  Circuit build timeouts are counted as construction failures if the
  circuit fails to complete before the 95% "right-censored" timeout
  interval, not the 80% timeout condition (see section 2.4).

  If a circuit closes prematurely after construction but before being
  requested to close by the client, this is counted as a failure.

7.2. Measuring path usage success rates

  Clients maintain two usage counts for each of their guards: a count
  of the number of usage attempts, and a count of the number of
  successful usages.

  A usage attempt means any attempt to attach a stream to a circuit.

  Usage success status is temporarily recorded by state flags on circuits.
  Guard usage success counts are not incremented until circuit close. A
  circuit is marked as successfully used if we receive a properly
  recognized RELAY cell on that circuit that was expected for the current
  circuit purpose.

  If subsequent stream attachments fail or time out, the successfully used
  state of the circuit is cleared, causing it once again to be regarded
  as a usage attempt only.

  Upon close by the client, all circuits that are still marked as usage
  attempts are probed using a RELAY_BEGIN cell constructed with a
  destination of the form 0.a.b.c:25, where a.b.c is a 24 bit random
  nonce. If we get a RELAY_COMMAND_END in response matching our nonce,
  the circuit is counted as successfully used.

  If any unrecognized RELAY cells arrive after the probe has been sent,
  the circuit is counted as a usage failure.

  If the stream failure reason codes DESTROY, TORPROTOCOL, or INTERNAL
  are received in response to any stream attempt, such circuits are not
  probed and are declared usage failures.

  Prematurely closed circuits are not probed, and are counted as usage
  failures.

7.3. Scaling success counts

  To provide a moving average of recent Guard activity while
  still preserving the ability to verify correctness, we periodically
  "scale" the success counts by multiplying them by a scale factor
  between 0 and 1.0.

  Scaling is performed when either usage or construction attempt counts
  exceed a parametrized value.

  To avoid error due to scaling during circuit construction and use,
  currently open circuits are subtracted from the usage counts before
  scaling, and added back after scaling.

7.4. Parametrization

   The following consensus parameters tune various aspects of the
   defense.

     pb_mincircs
       Default: 150
       Min: 5
       Effect: This is the minimum number of circuits that must complete
               at least 2 hops before we begin evaluating construction rates.


     pb_noticepct
       Default: 70
       Min: 0
       Max: 100
       Effect: If the circuit success rate falls below this percentage,
               we emit a notice log message.

     pb_warnpct
       Default: 50
       Min: 0
       Max: 100
       Effect: If the circuit success rate falls below this percentage,
               we emit a warn log message.

     pb_extremepct
       Default: 30
       Min: 0
       Max: 100
       Effect: If the circuit success rate falls below this percentage,
               we emit a more alarmist warning log message. If
               pb_dropguard is set to 1, we also disable the use of the
               guard.

     pb_dropguards
       Default: 0
       Min: 0
       Max: 1
       Effect: If the circuit success rate falls below pb_extremepct,
               when pb_dropguard is set to 1, we disable use of that
               guard.

     pb_scalecircs
       Default: 300
       Min: 10
       Effect: After this many circuits have completed at least two hops,
               Tor performs the scaling described in Section 7.3.

     pb_multfactor and pb_scalefactor
       Default: 1/2
       Min: 0.0
       Max: 1.0
       Effect: The double-precision result obtained from
               pb_multfactor/pb_scalefactor is multiplied by our current
               counts to scale them.

     pb_minuse
       Default: 20
       Min: 3
       Effect: This is the minimum number of circuits that we must attempt to
               use before we begin evaluating construction rates.

     pb_noticeusepct
       Default: 80
       Min: 3
       Effect: If the circuit usage success rate falls below this percentage,
               we emit a notice log message.

     pb_extremeusepct
       Default: 60
       Min: 3
       Effect: If the circuit usage success rate falls below this percentage,
               we emit a warning log message. We also disable the use of the
               guard if pb_dropguards is set.

     pb_scaleuse
       Default: 100
       Min: 10
       Effect: After we have attempted to use this many circuits,
               Tor performs the scaling described in Section 7.3.

7.5. Known barriers to enforcement

  Due to intermittent CPU overload at relays, the normal rate of
  successful circuit completion is highly variable. The Guard-dropping
  version of the defense is unlikely to be deployed until the ntor
  circuit handshake is enabled, or the nature of CPU overload induced
  failure is better understood.



X. Old notes

X.1. Do we actually do this?

How to deal with network down.
  - While all helpers are down/unreachable and there are no established
    or on-the-way testing circuits, launch a testing circuit. (Do this
    periodically in the same way we try to establish normal circuits
    when things are working normally.)
    (Testing circuits are a special type of circuit, that streams won't
    attach to by accident.)
  - When a testing circuit succeeds, mark all helpers up and hold
    the testing circuit open.
  - If a connection to a helper succeeds, close all testing circuits.
    Else mark that helper down and try another.
  - If the last helper is marked down and we already have a testing
    circuit established, then add the first hop of that testing circuit
    to the end of our helper node list, close that testing circuit,
    and go back to square one. (Actually, rather than closing the
    testing circuit, can we get away with converting it to a normal
    circuit and beginning to use it immediately?)

  [Do we actually do any of the above?  If so, let's spec it.  If not, let's
  remove it. -NM]

X.2. A thing we could do to deal with reachability.

And as a bonus, it leads to an answer to Nick's attack ("If I pick
my helper nodes all on 18.0.0.0:*, then I move, you'll know where I
bootstrapped") -- the answer is to pick your original three helper nodes
without regard for reachability. Then the above algorithm will add some
more that are reachable for you, and if you move somewhere, it's more
likely (though not certain) that some of the originals will become useful.
Is that smart or just complex?

X.3. Some stuff that worries me about entry guards. 2006 Jun, Nickm.

  It is unlikely for two users to have the same set of entry guards.
  Observing a user is sufficient to learn its entry guards.  So, as we move
  around, entry guards make us linkable.  If we want to change guards when
  our location (IP? subnet?) changes, we have two bad options.  We could

    - Drop the old guards.  But if we go back to our old location,
      we'll not use our old guards.  For a laptop that sometimes gets used
      from work and sometimes from home, this is pretty fatal.
    - Remember the old guards as associated with the old location, and use
      them again if we ever go back to the old location.  This would be
      nasty, since it would force us to record where we've been.

  [Do we do any of this now? If not, this should move into 099-misc or
  098-todo. -NM]
